This invention relates to a new method for preparing certain phosphorus-containing acids and salts.
The use of dialkyldithiophosphates and diaryldithiophosphates as additives in lubricating oils is well known. The diaryldithiophosphates have better thermal stability compared to the dialkyldithiophosphates. However, the dialkydithiophosphates have better extreme-pressure and hydrolytic stability compared to the diaryldithiophosphates.
The preparation of mixed aryl-alkyl dithiophosphates by methods previously known in the art is very difficult. For example, if a mixture of an alcohol and an aromatic phenol is reacted directly with phosphorus pentasulfide, the differences in the reactivity of the aromatic and alkyl alcohols results in the formation of very little of the mixed aryl-alkyl dithiophosphoric acid. If the salts (e.g., Zn salts) of the aryl dithiophosphate and the alkyl dithiophosphate are mixed, equilibration to a mixed aryl-alkyl dithiophosphate salt does not occur.